There is a need for a compact, low cost variable transmission that overcomes the limitations of the current variable belt technology. The belts are subject to external water immersion. There is also a need to have a modular concept that minimizes the re-design work needed to adopt hydromechanical technology. There is also a need for a design that can be easily tailored to different variable ratio range requirements that may be required for different vehicles. There is also a need to eliminate the need for a transmission range box in order to reduce overall cost and simplify operation of the vehicle, and to eliminate inconvenience of mechanical shifting.
There is also a need to take the operating mode decision away from the operator in order to improve machine performance and life. The need also exists for a low cost method to provide neutral and smooth startup of the vehicle.
It is therefore a principal object of this invention to provide a hydromechanical transmission that can be operated in either a hydromechanical mode or a hydrostatic mode.
A further object of this invention is to provide such a hydromechanical transmission wherein the hydromechanical mode enables vehicle operation throughout the complete forward speed range without any need for operator clutching or shifting, and the hydrostatic mode enables vehicle operation in both forward and reverse in a limited speed range, but without any need for operator clutching or shifting.
The transmission of this invention is designed to be a very compact integrated continuously variable transmission package. It can be used as a complete transmission that covers a wide range of speeds and torques without the need for an additional mechanical range box, or it can be used as a portion of a complete transmission system integrated with other components or systems of a vehicle drive train, without the need for an additional mechanical range box.
This invention involves a transmission with a single operating range without an additional mechanical range box. This will be more convenient to the vehicle operator, and will also eliminate the possibility of operating in an inappropriate gear range that could reduce life or performance of the vehicle.
Through the specific design of this transmission, this invention inherently provides the ability to achieve zero vehicle speed and smooth startup without the need and additional cost of added components common in the current state of the art. (No start-up clutch needed.) This transmission provides a continuously variable ratio between the input and output speeds and torques. This is accomplished through a hydromechanical transmission mode for driving forward, and a hydrostatic transmission mode for driving reverse, but also forward up to a certain speed less than maximum speed.
In the hydromechanical mode, as the variable hydrostatic unit strokes from maximum negative displacement to maximum positive displacement, the vehicle speed will vary continuously from zero to maximum forward. In the hydrostatic mode, as the variable hydrostatic unit strokes from maximum negative displacement to maximum positive displacement the vehicle speed will vary continuously from maximum reverse to partial forward speed. In the hydrostatic mode the transition from forward to reverse can be achieved without any shifting.
Changing from the hydromechanical mode to the hydrostatic mode is accomplished through a sliding clutch or friction clutch, which can be actuated mechanically, hydraulically, or electromechanically. In the case of a sliding, positive engagement type clutch, the vehicle may need to be stopped before changing the clutch position. The actuation of the clutch could be operator controlled, or could be automated through the electronic control system for this transmission. The clutch can also be used for the purpose of disengaging the entire transmission from the power input (engine), by putting the clutch into a neutral position.